Influence of membrane cholesterol on modulation of the GABA(A) receptor by neuroactive steroids and other potentiators

GABA and astrocytic cholesterol determine the lipid environment of GABAAR in cultured cortical neurons

The γ-aminobutyric acid (GABA) type A receptor (GABAAR), a GABA activated pentameric chloride channel, mediates fast inhibitory neurotransmission in the brain. The lipid environment is critical for GABAAR function. How lipids regulate the channel in the cell membrane is not fully understood. Here we employed super resolution imaging of lipids to demonstrate that the agonist GABA induces a rapid and reversible membrane translocation of GABAAR to phosphatidylinositol 4,5-bisphosphate (PIP2) clusters in mouse primary cortical neurons. This translocation relies on nanoscopic separation of PIP2 clusters and lipid rafts (cholesterol-dependent ganglioside clusters). In a resting state, the GABAAR associates with lipid rafts and this colocalization is enhanced by uptake of astrocytic secretions. These astrocytic secretions enhance endocytosis and delay desensitization. Our findings suggest intercellular signaling from astrocytes regulates GABAAR location based on lipid uptake in neurons. The findings have implications for treating mood disorders associated with altered neural excitability.

Structural insights into opposing actions of neurosteroids on GABAA receptors

γ-Aminobutyric acid type A (GABAA) receptors mediate fast inhibitory signaling in the brain and are targets of numerous drugs and endogenous neurosteroids. A subset of neurosteroids are GABAA receptor positive allosteric modulators; one of these, allopregnanolone, is the only drug approved specifically for treating postpartum depression. There is a consensus emerging from structural, physiological and photolabeling studies as to where positive modulators bind, but how they potentiate GABA activation remains unclear. Other neurosteroids are negative modulators of GABAA receptors, but their binding sites remain debated. Here we present structures of a synaptic GABAA receptor bound to allopregnanolone and two inhibitory sulfated neurosteroids. Allopregnanolone binds at the receptor-bilayer interface, in the consensus potentiator site. In contrast, inhibitory neurosteroids bind in the pore. MD simulations and electrophysiology support a mechanism by which allopregnanolone potentiates channel activity and suggest the dominant mechanism for sulfated neurosteroid inhibition is through pore block.

Cholesterol in Class C GPCRs: Role, Relevance, and Localization

G-protein coupled receptors (GPCRs), one of the largest superfamilies of cell-surface receptors, are heptahelical integral membrane proteins that play critical roles in virtually every organ system. G-protein-coupled receptors operate in membranes rich in cholesterol, with an imbalance in cholesterol level within the vicinity of GPCR transmembrane domains affecting the structure and/or function of many GPCRs, a phenomenon that has been linked to several diseases. These effects of cholesterol could result in indirect changes by altering the mechanical properties of the lipid environment or direct changes by binding to specific sites on the protein. There are a number of studies and reviews on how cholesterol modulates class A GPCRs; however, this area of study is yet to be explored for class C GPCRs, which are characterized by a large extracellular region and often form constitutive dimers. This review highlights specific sites of interaction, functions, and structural dynamics involved in the cholesterol recognition of the class C GPCRs. We summarize recent data from some typical family members to explain the effects of membrane cholesterol on the structural features and functions of class C GPCRs and speculate on their corresponding therapeutic potential.

Recent Insight into Lipid Binding and Lipid Modulation of Pentameric Ligand-Gated Ion Channels

Pentameric ligand-gated ion channels (pLGICs) play a leading role in synaptic communication, are implicated in a variety of neurological processes, and are important targets for the treatment of neurological and neuromuscular disorders. Endogenous lipids and lipophilic compounds are potent modulators of pLGIC function and may help shape synaptic communication. Increasing structural and biophysical data reveal sites for lipid binding to pLGICs. Here, we update our evolving understanding of pLGIC–lipid interactions highlighting newly identified modes of lipid binding along with the mechanistic understanding derived from the new structural data.

Neurosteroid Modulation of GABAA Receptor Function by Independent Action at Multiple Specific Binding Sites

Neurosteroids are endogenous modulators of GABA_A receptors that mediate anxiety, pain, mood and arousal. The 3-hydroxyl epimers, allopregnanolone (3α-OH) and epiallopregnanolone (3β-OH) are both prevalent in the mammalian brain and produce opposite effects on GABA_A receptor function, acting as positive and negative allosteric modulators, respectively. This Perspective provides a model to explain the actions of 3α-OH and 3β-OH neurosteroids. The model is based on evidence that the neurosteroid epimers bind to an overlapping subset of specific sites on GABA_A receptors, with their net functional effect on channel gating being the sum of their independent effects at each site.

Emerging role of HDL in brain cholesterol metabolism and neurodegenerative disorders

High-density lipoproteins (HDLs play a key role in cholesterol homeostasis maintenance in the central nervous system (CNS), by carrying newly synthesized cholesterol from astrocytes to neurons, to support their lipid-related physiological functions. As occurs for plasma HDLs, brain lipoproteins are assembled through the activity of membrane cholesterol transporters, undergo remodeling mediated by specific enzymes and transport proteins, and finally deliver cholesterol to neurons by a receptor-mediated internalization process. A growing number of evidences indicates a strong association between alterations of CNS cholesterol homeostasis and neurodegenerative disorders, in particular Alzheimer's disease (AD), and a possible role in this relationship may be played by defects in brain HDL metabolism. In the present review, we summarize and critically examine the current state of knowledge on major modifications of HDL and HDL-mediated brain cholesterol transport in AD, by taking into consideration the individual steps of this process. We also describe potential and encouraging HDL-based therapies that could represent new therapeutic strategies for AD treatment. Finally, we revise the main plasma and brain HDL modifications in other neurodegenerative disorders including Parkinson's disease (PD), Huntington's disease (HD), and frontotemporal dementia (FTD).

Druggable Lipid Binding Sites in Pentameric Ligand-Gated Ion Channels and Transient Receptor Potential Channels

Lipids modulate the function of many ion channels, possibly through direct lipid-protein interactions. The recent outpouring of ion channel structures by cryo-EM has revealed many lipid binding sites. Whether these sites mediate lipid modulation of ion channel function is not firmly established in most cases. However, it is intriguing that many of these lipid binding sites are also known sites for other allosteric modulators or drugs, supporting the notion that lipids act as endogenous allosteric modulators through these sites. Here, we review such lipid-drug binding sites, focusing on pentameric ligand-gated ion channels and transient receptor potential channels. Notable examples include sites for phospholipids and sterols that are shared by anesthetics and vanilloids. We discuss some implications of lipid binding at these sites including the possibility that lipids can alter drug potency or that understanding protein-lipid interactions can guide drug design. Structures are only the first step toward understanding the mechanism of lipid modulation at these sites. Looking forward, we identify knowledge gaps in the field and approaches to address them. These include defining the effects of lipids on channel function in reconstituted systems using asymmetric membranes and measuring lipid binding affinities at specific sites using native mass spectrometry, fluorescence binding assays, and computational approaches.

Statins: Neurobiological underpinnings and mechanisms in mood disorders

Statins (3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors) treat dyslipidaemia and cardiovascular disease by inhibiting cholesterol biosynthesis. They also have immunomodulatory and anti-inflammatory properties. Beyond cardiovascular disease, cholesterol and inflammation appear to be components of the pathogenesis and pathophysiology of neuropsychiatric disorders. Statins may therefore afford some therapeutic benefit in mood disorders. In this paper, we review the pathophysiology of mood disorders with a focus on pharmacologically relevant pathways, using major depressive disorder and bipolar disorder as exemplars. Statins are discussed in the context of these disorders, with particular focus on the putative mechanisms involved in their anti-inflammatory and immunomodulatory effects. Recent clinical data suggest that statins may have antidepressant properties, however given their interactions with many known biological pathways, it has not been fully elucidated which of these are the major determinants of clinical outcomes in mood disorders. Moreover, it remains unclear what the appropriate dose, or appropriate patient phenotype for adjunctive treatment may be. High quality randomised control trials in concert with complementary biological investigations are needed if the potential clinical effects of statins on mood disorders, as well as their biological correlates, are to be better understood.

Interfacial binding sites for cholesterol on GABAA receptors and competition with neurosteroids

γ-Aminobutyric acid type A (GABA_A) receptors in the brain are located in the outer membranes of brain cells where the concentration of cholesterol is high. Of the 25 available high-resolution structures available for GABA_A receptors, none were determined in the presence of cholesterol, but four include resolved molecules of cholesterol hemisuccinate (CHS). Here, a molecular docking procedure is used to sweep the transmembrane (TM) surfaces of the receptors for cholesterol binding sites. Cholesterol docking poses determined in this way match 89% of the resolved CHS when CHS molecules deemed unlikely to represent typical bound cholesterols are excluded. The receptors are pentameric, and their TM surfaces consist of a set of five facets, each including pairs of TM helices from two adjacent subunits. Each facet contains hydrophobic hollows running from one side of the membrane to the other, within which are six potential binding sites for cholesterol, three on each side of the membrane. High-resolution structures of GABA_A receptors with bound neurosteroids show that neurosteroids bind in these cholesterol binding sites, so the binding of neurosteroids and cholesterol will be competitive.

Functional marriage in plasma membrane: Critical cholesterol level-optimal protein activity

In physiology, homeostasis refers to the condition where a system exhibits an optimum functional level. In contrast, any variation from this optimum is considered as a dysfunctional or pathological state. In this review, we address the proposal that a critical cholesterol level in the plasma membrane is required for the proper functioning of transmembrane proteins. Thus, membrane cholesterol depletion or enrichment produces a loss or gain of direct cholesterol-protein interaction and/or changes in the physical properties of the plasma membrane, which affect the basal or optimum activity of transmembrane proteins. Whether or not this functional switching is a generalized mechanism exhibited for all transmembrane proteins, or if it works just for an exclusive group of them, is an open question and an attractive subject to explore at a basic, pharmacological and clinical level.

Electrophysiological evaluation of extracellular spermine and alkaline pH on synaptic human GABAA receptors

Polyamines have fundamental roles in brain homeostasis as key modulators of cellular excitability. Several studies have suggested alterations in polyamine metabolism in stress related disorders, suicide, depression, and neurodegeneration, making the pharmacological modulation of polyamines a highly appealing therapeutic strategy. Polyamines are small aliphatic molecules that can modulate cationic channels involved in neuronal excitability. Previous indirect evidence has suggested that polyamines can modulate anionic GABA_A receptors (GABA_ARs), which mediate inhibitory signaling and provide a direct route to reduce hyperexcitability. Here, we attempted to characterize the effect that spermine, the polyamine with the strongest reported effect on GABA_ARs, has on human postmortem native GABA_ARs. We microtransplanted human synaptic membranes from the dorsolateral prefrontal cortex of four cases with no history of mental or neurological disorders, and directly recorded spermine effects on ionic GABA_ARs responses on microtransplanted oocytes. We show that in human synapses, inhibition of GABA_ARs by spermine was better explained by alkalization of the extracellular solution. Additionally, spermine had no effect on the potentiation of GABA-currents by diazepam, indicating that even if diazepam binding is enhanced by spermine, it does not translate to changes in functional activity. Our results clearly demonstrate that while extracellular spermine does not have direct effects on human native synaptic GABA_ARs, spermine-mediated shifts of pH inhibit GABA_ARs. Potential spermine-mediated increase of pH in synapses in vivo may therefore participate in increased neuronal activity observed during physiological and pathological states, and during metabolic alterations that increase the release of spermine to the extracellular milieu.

Cholesterol Regulation of Pulmonary Endothelial Calcium Homeostasis

Cholesterol is a key structural component and regulator of lipid raft signaling platforms critical for cell function. Such regulation may involve changes in the biophysical properties of lipid microdomains or direct protein-sterol interactions that alter the function of ion channels, receptors, enzymes, and membrane structural proteins. Recent studies have implicated abnormal membrane cholesterol levels in mediating endothelial dysfunction that is characteristic of pulmonary hypertensive disorders, including that resulting from long-term exposure to hypoxia. Endothelial dysfunction in this setting is characterized by impaired pulmonary endothelial calcium entry and an associated imbalance that favors production vasoconstrictor and mitogenic factors that contribute to pulmonary hypertension. Here we review current knowledge of cholesterol regulation of pulmonary endothelial Ca²⁺ homeostasis, focusing on the role of membrane cholesterol in mediating agonist-induced Ca²⁺ entry and its components in the normal and hypertensive pulmonary circulation.

State-dependent alterations of lipid profiles in patients with bipolar disorder

Objective Serum lipid levels may be associated with the affective severity of bipolar disorder, but data on lipid profiles in Asian patients with bipolar disorder and the lipid alterations in different states of opposite polarities are scant. We investigated the lipid profiles of patients in the acute affective, partial, and full remission state in bipolar mania and depression. Methods The physically healthy patients aged between 18 and 45 years with bipolar I disorder, as well as age-matched healthy normal controls were enrolled. We compared the fasting blood levels of glucose, cholesterol, triglyceride, low-density lipoprotein, and high-density lipoprotein of manic or depressed patients in the acute phase and subsequent partial and full remission with those of their normal controls. Results A total of 32 bipolar manic patients (12 women and 20 men), 32 bipolar depressed participants (18 women and 14 men), and 64 healthy control participants took part in this study. The mean cholesterol level in acute mania was significantly lower than that in acute depression (p < 0.025). The lowest rate of dyslipidemia (hypertriglyceridemia or low high-density lipoprotein cholesterol) was observed in acute bipolar mania. Conclusion Circulating lipid profiles may be easily affected by affective states. The acute manic state may be accompanied by state-dependent lower cholesterol and triglyceride levels relative to that in other mood states.

Reduced membrane cholesterol after chronic hypoxia limits Orai1-mediated pulmonary endothelial Ca2+ entry

Endothelial dysfunction in chronic hypoxia (CH)-induced pulmonary hypertension is characterized by reduced store-operated Ca²⁺ entry (SOCE) and diminished Ca²⁺-dependent production of endothelium-derived vasodilators. We recently reported that SOCE in pulmonary arterial endothelial cells (PAECs) is tightly regulated by membrane cholesterol and that decreased membrane cholesterol is responsible for impaired SOCE after CH. However, the ion channels involved in cholesterol-sensitive SOCE are unknown. We hypothesized that cholesterol facilitates SOCE in PAECs through the interaction of Orai1 and stromal interaction molecule 1 (STIM1). The role of cholesterol in Orai1-mediated SOCE was initially assessed using CH exposure in rats (4 wk, 380 mmHg) as a physiological stimulus to decrease PAEC cholesterol. The effects of Orai1 inhibition with AnCoA4 on SOCE were examined in isolated PAEC sheets from control and CH rats after cholesterol supplementation, substitution of endogenous cholesterol with epicholesterol (Epichol), or vehicle treatment. Whereas cholesterol restored endothelial SOCE in CH rats, both Epichol and AnCoA4 attenuated SOCE only in normoxic controls. The Orai1 inhibitor had no further effect in cells pretreated with Epichol. Using cultured pulmonary endothelial cells to allow better mechanistic analysis of the molecular components of cholesterol-regulated SOCE, we found that Epichol, AnCoA4, and Orai1 siRNA each inhibited SOCE compared with their respective controls. Epichol had no additional effect after knockdown of Orai1. Furthermore, Epichol substitution significantly reduced STIM1-Orai1 interactions as assessed by a proximity ligation assay. We conclude that membrane cholesterol is required for the STIM1-Orai1 interaction necessary to elicit endothelial SOCE. Furthermore, reduced PAEC membrane cholesterol after CH limits Orai1-mediated SOCE. NEW & NOTEWORTHY This research demonstrates a novel contribution of cholesterol to regulate the interaction of Orai1 and stromal interaction molecule 1 required for pulmonary endothelial store-operated Ca²⁺ entry. The results provide a mechanistic basis for impaired pulmonary endothelial Ca²⁺ influx after chronic hypoxia that may contribute to pulmonary hypertension.

Common structural features of cholesterol binding sites in crystallized soluble proteins

Cholesterol-protein interactions are essential for the architectural organization of cell membranes and for lipid metabolism. While cholesterol-sensing motifs in transmembrane proteins have been identified, little is known about cholesterol recognition by soluble proteins. We reviewed the structural characteristics of binding sites for cholesterol and cholesterol sulfate from crystallographic structures available in the Protein Data Bank. This analysis unveiled key features of cholesterol-binding sites that are present in either all or the majority of sites: i) the cholesterol molecule is generally positioned between protein domains that have an organized secondary structure; ii) the cholesterol hydroxyl/sulfo group is often partnered by Asn, Gln, and/or Tyr, while the hydrophobic part of cholesterol interacts with Leu, Ile, Val, and/or Phe; iii) cholesterol hydrogen-bonding partners are often found on α-helices, while amino acids that interact with cholesterol's hydrophobic core have a slight preference for β-strands and secondary structure-lacking protein areas; iv) the steroid's C21 and C26 constitute the "hot spots" most often seen for steroid-protein hydrophobic interactions; v) common "cold spots" are C8-C10, C13, and C17, at which contacts with the proteins were not detected. Several common features we identified for soluble protein-steroid interaction appear evolutionarily conserved.

Structural Stringency of Cholesterol for Membrane Protein Function Utilizing Stereoisomers as Novel Tools: A Review

Cholesterol is an important lipid in the context of membrane protein function. The function of a number of membrane proteins, including G protein-coupled receptors (GPCRs) and ion channels, has been shown to be dependent on membrane cholesterol. However, the molecular mechanism underlying such regulation is still being explored. In some cases, specific interaction between cholesterol and the protein has been implicated. In other cases, the effect of cholesterol on the membrane properties has been attributed for the regulation of protein function. In this article, we have provided an overview of experimental approaches that are useful for determining the degree of structural stringency of cholesterol for membrane protein function. In the process, we have highlighted the role of immediate precursors in cholesterol biosynthetic pathway in the function of membrane proteins. Special emphasis has been given to the application of stereoisomers of cholesterol in deciphering the structural stringency required for regulation of membrane protein function. A comprehensive examination of these processes would help in understanding the molecular basis of cholesterol regulation of membrane proteins in subtle details.

Distinct mechanisms underlying cholesterol protection against alcohol-induced BK channel inhibition and resulting vasoconstriction

Alcohol (ethanol) at concentrations reached in blood following moderate to heavy drinking (30-80mM) reduces cerebral artery diameter via inhibition of voltage- and calcium-gated potassium channels of large conductance (BK) in cerebral artery smooth muscle. These channels consist of channel-forming α and regulatory β1 subunits. A high-cholesterol diet protects against ethanol-induced constriction via accumulation of cholesterol within the vasculature. The molecular mechanisms of this protection remain unknown. In the present work, we demonstrate that in vitro cholesterol enrichment of rat middle cerebral arteries significantly increased cholesterol within arterial tissues and blunted constriction by 50mM of ethanol. Ethanol-induced BK channel inhibition in inside-out patches excised from freshly isolated cerebral artery myocytes was also abolished by cholesterol enrichment. Enrichment of arteries with enantiomeric cholesterol (ent-cholesterol) also blunted BK channel inhibition and cerebral artery constriction in response to ethanol. The similar protection of cholesterol and ent-cholesterol against ethanol action indicates that this protection does not require protein site(s) that specifically sense natural cholesterol. Cholesterol-driven protection against ethanol-induced BK channel inhibition and vasoconstriction was replicated in myocytes and middle cerebral arteries of C57BL/6 mice. BK β1 subunits are known to regulate vascular diameter and its modification by ethanol. However, blunting of an ethanol effect by in vitro cholesterol enrichment was observed in arteries and myocyte membrane patches from BK β1 (KCNMB1) knockout mice. Thus, BK β1 subunits are not needed for cholesterol protection against ethanol effect on BK channel function and cerebral artery diameter.

The lipid habitats of neurotransmitter receptors in brain

Neurotransmitter receptors, the macromolecules specialized in decoding the chemical signals encrypted in the chemical signaling mechanism in the nervous system, occur either at the somatic cell surface of chemically excitable cells or at specialized subcellular structures, the synapses. Synapses have lipid compositions distinct from the rest of the cell membrane, suggesting that neurotransmitter receptors and their scaffolding and adaptor protein partners require specific lipid habitats for optimal operation. In this review we discuss some paradigmatic cases of neurotransmitter receptor-lipid interactions, highlighting the chemical nature of the intervening lipid species and providing examples of the receptor mechanisms affected by interaction with lipids. The focus is on the effects of cholesterol, glycerophospholipids and covalent fatty acid acylation on neurotransmitter receptors. We also briefly discuss the role of lipid phase states involving lateral heterogeneities of the host membrane known to modulate membrane transport, protein sorting and signaling. Modulation of neurotransmitter receptors by lipids occurs at multiple levels, affecting a wide span of activities including their trafficking, sorting, stability, residence lifetime at the cell surface, endocytosis, and recycling, among other important functional properties at the synapse.

Membrane Protein Structure, Function, and Dynamics: a Perspective from Experiments and Theory

Membrane proteins mediate processes that are fundamental for the flourishing of biological cells. Membrane-embedded transporters move ions and larger solutes across membranes; receptors mediate communication between the cell and its environment and membrane-embedded enzymes catalyze chemical reactions. Understanding these mechanisms of action requires knowledge of how the proteins couple to their fluid, hydrated lipid membrane environment. We present here current studies in computational and experimental membrane protein biophysics, and show how they address outstanding challenges in understanding the complex environmental effects on the structure, function, and dynamics of membrane proteins.

Risk factors for late-onset generalized anxiety disorder: results from a 12-year prospective cohort (the ESPRIT study)

Generalized anxiety disorder (GAD) is a chronic and highly prevalent disorder associated with increased disability and mortality in the elderly. Treatment is difficult with low rate of full remission, thus highlighting the need to identify early predictors for prevention in elderly people. The aim of this study is to identify and characterize incident GAD predictors in elderly people. A total of 1711 individuals aged 65 years and above and free of GAD at baseline were randomly recruited from electoral rolls between 1999 and 2001 (the prospective ESPRIT study). The participants were examined at baseline and five times over 12 years. GAD and psychiatric comorbidity were diagnosed with a standardized psychiatric examination, the Mini-International Neuropsychiatry Interview on the basis of DSM-IV (Diagnostic and Statistical Manual of Mental Disorders, fourth edition) criteria and validated by a clinical panel. During the follow-up, 8.4% (95% confidence interval=7.1-9.7%) of the participants experienced incident GAD, 80% being first episodes; the incident rate being 10 per 1000 person-years. The principal predictors of late-onset incident GAD over 12 years derived from a multivariate Cox model were being female, recent adverse life events, having chronic physical (respiratory disorders, arrhythmia and heart failure, dyslipidemia, cognitive impairment) and mental (depression, phobia and past GAD) health disorders. Poverty, parental loss or separation and low affective support during childhood, as well as history of mental problems in parents were also significantly and independently associated with incident GAD. GAD appears as a multifactorial stress-related affective disorder resulting from both proximal and distal risk factors, some of them being potentially modifiable by health care intervention.

Using Sterol Substitution to Probe the Role of Membrane Domains in Membrane Functions

Ordered membrane lipid domains rich in sphingolipids and sterols ("lipid rafts") are thought to be important in many biological processes.  However, it is often difficult to distinguish domain-dependent biological functions from ones that have a specific dependence on sterol, e.g. are dependent upon a protein with a function that is dependent upon its binding to sterol.  Removing cholesterol and replacing it with various sterols with varying abilities to form membrane domains or otherwise alter membrane properties has the potential to help distinguish these cases.  This review describes this strategy, and how it has been applied by various investigators to understand cellular functions.

Effect of neurosteroids on a model lipid bilayer including cholesterol: An Atomic Force Microscopy study

Amphiphilic molecules which have a biological effect on specific membrane proteins, could also affect lipid bilayer properties possibly resulting in a modulation of the overall membrane behavior. In light of this consideration, it is important to study the possible effects of amphiphilic molecule of pharmacological interest on model systems which recapitulate some of the main properties of the biological plasma membranes. In this work we studied the effect of a neurosteroid, Allopregnanolone (3α,5α-tetrahydroprogesterone or Allo), on a model bilayer composed by the ternary lipid mixture DOPC/bSM/chol. We chose ternary mixtures which present, at room temperature, a phase coexistence of liquid ordered (Lo) and liquid disordered (Ld) domains and which reside near to a critical point. We found that Allo, which is able to strongly partition in the lipid bilayer, induces a marked increase in the bilayer area and modifies the relative proportion of the two phases favoring the Ld phase. We also found that the neurosteroid shifts the miscibility temperature to higher values in a way similarly to what happens when the cholesterol concentration is decreased. Interestingly, an isoform of Allo, isoAllopregnanolone (3β,5α-tetrahydroprogesterone or isoAllo), known to inhibit the effects of Allo on GABAA receptors, has an opposite effect on the bilayer properties.

A predicted binding site for cholesterol on the GABAA receptor

Modulation of the GABA type A receptor (GABAAR) function by cholesterol and other steroids is documented at the functional level, yet its structural basis is largely unknown. Current data on structurally related modulators suggest that cholesterol binds to subunit interfaces between transmembrane domains of the GABAAR. We construct homology models of a human GABAAR based on the structure of the glutamate-gated chloride channel GluCl of Caenorhabditis elegans. The models show the possibility of previously unreported disulfide bridges linking the M1 and M3 transmembrane helices in the α and γ subunits. We discuss the biological relevance of such disulfide bridges. Using our models, we investigate cholesterol binding to intersubunit cavities of the GABAAR transmembrane domain. We find that very similar binding modes are predicted independently by three approaches: analogy with ivermectin in the GluCl crystal structure, automated docking by AutoDock, and spontaneous rebinding events in unbiased molecular dynamics simulations. Taken together, the models and atomistic simulations suggest a somewhat flexible binding mode, with several possible orientations. Finally, we explore the possibility that cholesterol promotes pore opening through a wedge mechanism.

Liquid general anesthetics lower critical temperatures in plasma membrane vesicles

A large and diverse array of small hydrophobic molecules induce general anesthesia. Their efficacy as anesthetics has been shown to correlate both with their affinity for a hydrophobic environment and with their potency in inhibiting certain ligand-gated ion channels. In this study we explore the effects that n-alcohols and other liquid anesthetics have on the two-dimensional miscibility critical point observed in cell-derived giant plasma membrane vesicles (GPMVs). We show that anesthetics depress the critical temperature (Tc) of these GPMVs without strongly altering the ratio of the two liquid phases found below Tc. The magnitude of this affect is consistent across n-alcohols when their concentration is rescaled by the median anesthetic concentration (AC50) for tadpole anesthesia, but not when plotted against the overall concentration in solution. At AC50 we see a 4°C downward shift in Tc, much larger than is typically seen in the main chain transition at these anesthetic concentrations. GPMV miscibility critical temperatures are also lowered to a similar extent by propofol, phenylethanol, and isopropanol when added at anesthetic concentrations, but not by tetradecanol or 2,6 diterbutylphenol, two structural analogs of general anesthetics that are hydrophobic but have no anesthetic potency. We propose that liquid general anesthetics provide an experimental tool for lowering critical temperatures in plasma membranes of intact cells, which we predict will reduce lipid-mediated heterogeneity in a way that is complimentary to increasing or decreasing cholesterol. Also, several possible implications of our results are discussed in the context of current models of anesthetic action on ligand-gated ion channels.

Multiple propofol-binding sites in a γ-aminobutyric acid type A receptor (GABAAR) identified using a photoreactive propofol analog

Propofol acts as a positive allosteric modulator of γ-aminobutyric acid type A receptors (GABAARs), an interaction necessary for its anesthetic potency in vivo as a general anesthetic. Identifying the location of propofol-binding sites is necessary to understand its mechanism of GABAAR modulation. [(3)H]2-(3-Methyl-3H-diaziren-3-yl)ethyl 1-(phenylethyl)-1H-imidazole-5-carboxylate (azietomidate) and R-[(3)H]5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl)barbituric acid (mTFD-MPAB), photoreactive analogs of 2-ethyl 1-(phenylethyl)-1H-imidazole-5-carboxylate (etomidate) and mephobarbital, respectively, have identified two homologous but pharmacologically distinct classes of intersubunit-binding sites for general anesthetics in the GABAAR transmembrane domain. Here, we use a photoreactive analog of propofol (2-isopropyl-5-[3-(trifluoromethyl)-3H-diazirin-3-yl]phenol ([(3)H]AziPm)) to identify propofol-binding sites in heterologously expressed human α1β3 GABAARs. Propofol, AziPm, etomidate, and R-mTFD-MPAB each inhibited [(3)H]AziPm photoincorporation into GABAAR subunits maximally by ∼ 50%. When the amino acids photolabeled by [(3)H]AziPm were identified by protein microsequencing, we found propofol-inhibitable photolabeling of amino acids in the β3-α1 subunit interface (β3Met-286 in β3M3 and α1Met-236 in α1M1), previously photolabeled by [(3)H]azietomidate, and α1Ile-239, located one helical turn below α1Met-236. There was also propofol-inhibitable [(3)H]AziPm photolabeling of β3Met-227 in βM1, the amino acid in the α1-β3 subunit interface photolabeled by R-[(3)H]mTFD-MPAB. The propofol-inhibitable [(3)H]AziPm photolabeling in the GABAAR β3 subunit in conjunction with the concentration dependence of inhibition of that photolabeling by etomidate or R-mTFD-MPAB also establish that each anesthetic binds to the homologous site at the β3-β3 subunit interface. These results establish that AziPm as well as propofol bind to the homologous intersubunit sites in the GABAAR transmembrane domain that binds etomidate or R-mTFD-MPAB with high affinity.

Pregnenolone sulfate: from steroid metabolite to TRP channel ligand

Pregnenolone sulfate is a steroid metabolite with a plethora of actions and functions. As a neurosteroid, pregnenolone sulfate modulates a variety of ion channels, transporters, and enzymes. Interestingly, as a sulfated steroid, pregnenolone sulfate is not the final- or waste-product of pregnenolone being sulfated via a phase II metabolism reaction and renally excreted, as one would presume from the pharmacology textbook knowledge. Pregnenolone sulfate is also the source and thereby the starting point for subsequent steroid synthesis pathways. Most recently, pregnenolone sulfate has been functionally “upgraded” from modulator of ion channels to an activating ion channel ligand. This review will focus on molecular aspects of the neurosteroid, pregnenolone sulfate, its metabolism, concentrations in serum and tissues and last not least will summarize the functional data.

Large conductance, calcium- and voltage-gated potassium (BK) channels: regulation by cholesterol

Cholesterol (CLR) is an essential component of eukaryotic plasma membranes. CLR regulates the membrane physical state, microdomain formation and the activity of membrane-spanning proteins, including ion channels. Large conductance, voltage- and Ca²⁺-gated K⁺ (BK) channels link membrane potential to cell Ca²⁺ homeostasis. Thus, they control many physiological processes and participate in pathophysiological mechanisms leading to human disease. Because plasmalemma BK channels cluster in CLR-rich membrane microdomains, a major driving force for studying BK channel-CLR interactions is determining how membrane CLR controls the BK current phenotype, including its pharmacology, channel sorting, distribution, and role in cell physiology. Since both BK channels and CLR tissue levels play a pathophysiological role in human disease, identifying functional and structural aspects of the CLR-BK channel interaction may open new avenues for therapeutic intervention. Here, we review the studies documenting membrane CLR-BK channel interactions, dissecting out the many factors that determine the final BK current response to changes in membrane CLR content. We also summarize work in reductionist systems where recombinant BK protein is studied in artificial lipid bilayers, which documents a direct inhibition of BK channel activity by CLR and builds a strong case for a direct interaction between CLR and the BK channel-forming protein. Bilayer lipid-mediated mechanisms in CLR action are also discussed. Finally, we review studies of BK channel function during hypercholesterolemia, and underscore the many consequences that the CLR-BK channel interaction brings to cell physiology and human disease.

Modulation of presynaptic GABA(A) receptors by endogenous neurosteroids

BACKGROUND AND PURPOSE

Although 3α-hydroxy, 5α-reduced pregnane steroids, such as allopregnanolone (AlloP) and tetrahydrodeoxycorticosterone, are endogenous positive modulators of postsynaptic GABA(A) receptors, the functional roles of endogenous neurosteroids in synaptic transmission are still largely unknown.

EXPERIMENTAL APPROACH

In this study, the effect of AlloP on spontaneous glutamate release was examined in mechanically isolated dentate gyrus hilar neurons by use of the conventional whole-cell patch-clamp technique.

KEY RESULTS

AlloP increased the frequency of glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) in a dose-dependent manner. The AlloP-induced increase in sEPSC frequency was completely blocked by a non-competitive GABA(A) receptor blocker, tetrodotoxin or Cd(2+) , suggesting that AlloP acts on presynaptic GABA(A) receptors to depolarize presynaptic nerve terminals to increase the probability of spontaneous glutamate release. On the other hand, γ-cyclodextrin (γ-CD) significantly decreased the basal frequency of sEPSCs. However, γ-CD failed to decrease the basal frequency of sEPSCs in the presence of a non-competitive GABA(A) receptor antagonist or tetrodotoxin. In addition, γ-CD failed to decrease the basal frequency of sEPSCs after blocking the synthesis of endogenous 5α-reduced pregnane steroids. Furthermore, γ-CD decreased the extent of muscimol-induced increase in sEPSC frequency, suggesting that endogenous neurosteroids can directly activate and/or potentiate presynaptic GABA(A) receptors to affect spontaneous glutamate release onto hilar neurons.

CONCLUSIONS AND IMPLICATIONS

The modulation of presynaptic GABA(A) receptors by endogenous neurosteroids might affect the excitability of the dentate gyrus-hilus-CA3 network, and thus contribute, at least in part, to some pathological conditions, such as catamenial epilepsy and premenstrual dysphoric disorder.

Coupling between GABA(A)-R ligand-binding activity and membrane organization in β-cyclodextrin-treated synaptosomal membranes from bovine brain cortex: new insights from EPR experiments

Correlations between GABA(A) receptor (GABA(A)-R) activity and molecular organization of synaptosomal membranes (SM) were studied along the protocol for cholesterol (Cho) extraction with β-cyclodextrin (β-CD). The mere pre-incubation (PI) at 37°C accompanying the β-CD treatment was an underlying source of perturbations increasing [(3)H]-FNZ maximal binding (70%) and K (d) (38%), plus a stiffening of SMs' hydrocarbon core region. The latter was inferred from an increased compressibility modulus (K) of SM-derived Langmuir films, a blue-shifted DPH fluorescence emission spectrum and the hysteresis in DPH fluorescence anisotropy (A (DPH)) in SMs submitted to a heating-cooling cycle (4-37-4°C) with A (DPH,heating) < A (DPH,cooling). Compared with PI samples, the β-CD treatment reduced B (max) by 5% which correlated with a 45%-decrement in the relative Cho content of SM, a decrease in K and in the order parameter in the EPR spectrum of a lipid spin probe labeled at C5 (5-SASL), and significantly increased A (TMA-DPH). PI, but not β-CD treatment, could affect the binding affinity. EPR spectra of 5-SASL complexes with β-CD-, SM-partitioned, and free in solution showed that, contrary to what is usually assumed, β-CD is not completely eliminated from the system through centrifugation washings. It was concluded that β-CD treatment involves effects of at least three different types of events affecting membrane organization: (a) effect of PI on membrane annealing, (b) effect of residual β-CD on SM organization, and (c) Cho depletion. Consequently, molecular stiffness increases within the membrane core and decreases near the polar head groups, leading to a net increase in GABA(A)-R density, relative to untreated samples.

Smooth muscle cholesterol enables BK β1 subunit-mediated channel inhibition and subsequent vasoconstriction evoked by alcohol

OBJECTIVE

Hypercholesterolemia and alcohol drinking constitute independent risk factors for cerebrovascular disease. Alcohol constricts cerebral arteries in several species, including humans. This action results from inhibition of voltage- and calcium-gated potassium channels (BK) in vascular smooth muscle cells (VSMC). BK activity is also modulated by membrane cholesterol. We investigated whether VSMC cholesterol regulates ethanol actions on BK and cerebral arteries.

METHODS AND RESULTS

After myogenic tone development, cholesterol depletion of rat, resistance-size cerebral arteries ablated ethanol-induced constriction, a result that was identical in intact and endothelium-free vessels. Cholesterol depletion reduced ethanol inhibition of BK whether the channel was studied in VSMC or after rat cerebral artery myocyte subunit (cbv1+β1) reconstitution into phospholipid bilayers. Homomeric cbv1 channels reconstituted into bilayers and VSMC BK from β1 knockout mice were both resistant to ethanol-induced inhibition. Moreover, arteries from β1 knockout mice failed to respond to ethanol even when VSMC cholesterol was kept unmodified. Remarkably, ethanol inhibition of cbv1+β1 in bilayers and wt mouse VSMC BK were drastically blunted by cholesterol depletion. Consistently, cholesterol depletion suppressed ethanol constriction of wt mouse arteries.

CONCLUSION

VSMC cholesterol and BK β1 are both required for ethanol inhibition of BK and the resulting cerebral artery constriction, with health-related implications for manipulating cholesterol levels in alcohol-induced cerebrovascular disease.

Impact of lipid raft integrity on 5-HT3 receptor function and its modulation by antidepressants

Because of the biochemical colocalization of the 5-HT(3) receptor and antidepressants within raft-like domains and their antagonistic effects at this ligand-gated ion channel, we investigated the impact of lipid raft integrity for 5-HT(3) receptor function and its modulation by antidepressants. Treatment with methyl-beta-cyclodextrine (MbetaCD) markedly reduced membrane cholesterol levels and caused a more diffuse membrane distribution of the lipid raft marker protein flotillin-1 indicating lipid raft impairment. Both amplitude and charge of serotonin evoked cation currents were diminished following cholesterol depletion by either MbetaCD or simvastatin (Sim), whereas the functional antagonistic properties of the antidepressants desipramine (DMI) and fluoxetine (Fluox) at the 5-HT(3) receptor were retained. Although both the 5-HT(3) receptor and flotillin-1 were predominantly found in raft-like domains in western blots following sucrose density gradient centrifugation, immunocytochemistry revealed only a coincidental degree of colocalization of these two proteins. These findings and the persistence of the antagonistic effects of DMI and Fluox against 5-HT(3) receptors after lipid raft impairment indicate that their modulatory effects are likely mediated through non-raft 5-HT(3) receptors, which are not sufficiently detected by means of sucrose density gradient centrifugation. In conclusion, lipid raft integrity appears to be important for 5-HT(3) receptor function in general, whereas it is not a prerequisite for the antagonistic properties of antidepressants such as DMI and Fluox at this ligand-gated ion channel.

Cholesterol and affective morbidity

Depression and mania have been linked with low cholesterol though there has been limited prospective study of cholesterol and subsequent course of affective illness. We studied the relationship between fasting total cholesterol and subsequent depressive and manic symptoms. A total of 131 participants from a prospective cohort study were identified as having had a fasting total cholesterol evaluation at intake. Participants were predominantly inpatients at index visit and were followed for a median of 20 and up to 25 years. Cholesterol was modeled with age, gender, and index use of a mood stabilizer in linear regression to assess its influence on subsequent depressive symptom burden in participants with unipolar disorder as well as depressive and manic symptom burden in participants with bipolar disorder. Among bipolar participants (N=65), low cholesterol predicted a higher proportion of follow-up weeks with manic, but not depressive symptoms. Cholesterol did not appear to predict depressive symptom burden among participants with unipolar depression (N=66). Lower cholesterol levels may predispose individuals with bipolar disorder to a greater burden of manic symptomatology and may provide some insight into the underlying neurobiology.

A surface active benzodiazepine receptor ligand for potential probing membrane order of GABAA-receptor surroundings

A conjugable analogue of the benzodiazepine 5-(2-hydroxiphenyl)-7-nitro-benzo[ e][1,4]diazepin-2(3 H)-one N 1-substituted with an aliphatic chain (CNZ acyl derivative, CAd) was synthesized. CAd inhibited FNZ binding to GABA A-R with an inhibition binding constant K i = 176 nM and expanded a model membrane packed up to 13 mN/m when penetrating from the aqueous phase. CAd exhibited surface activity with a collapse pressure pi = 18.8 mN/m and minimal molecular area A min = 49 A (2)/molecule at the closest molecular packing, resulting in full and nonideal mixing with a phospholipid in a monolayer up to a molar fraction x congruent with 0.1, decreasing its surface potential and contributing with a dipole that pointed its positive end toward the air and reoriented at the interface upon compression. These findings suggested that CAd could be stabilized at the membrane-water interface with its CNZ moiety stacked at the GABA A-R while its acyl chain can be inserted into the membrane depth.

Two pools of Triton X-100-insoluble GABA(A) receptors are present in the brain, one associated to lipid rafts and another one to the post-synaptic GABAergic complex

Rat forebrain synaptosomes were extracted with Triton X-100 at 4 degrees C and the insoluble material, which is enriched in post-synaptic densities (PSDs), was subjected to sedimentation on a continuous sucrose gradient. Two pools of Triton X-100-insoluble gamma-aminobutyric acid type-A receptors (GABA(A)Rs) were identified: (i) a higher-density pool (rho = 1.10-1.15 mg/mL) of GABA(A)Rs that contains the gamma2 subunit (plus alpha and beta subunits) and that is associated to gephyrin and the GABAergic post-synaptic complex and (ii) a lower-density pool (rho = 1.06-1.09 mg/mL) of GABA(A)Rs associated to detergent-resistant membranes (DRMs) that contain alpha and beta subunits but not the gamma2 subunit. Some of these GABA(A)Rs contain the delta subunit. Two pools of GABA(A)Rs insoluble in Triton X-100 at 4 degrees C were also identified in cultured hippocampal neurons: (i) a GABA(A)R pool that forms clusters that co-localize with gephyrin and remains Triton X-100-insoluble after cholesterol depletion and (ii) a GABA(A)R pool that is diffusely distributed at the neuronal surface that can be induced to form GABA(A)R clusters by capping with an anti-alpha1 GABA(A)R subunit antibody and that becomes solubilized in Triton X-100 at 4 degrees C after cholesterol depletion. Thus, there is a pool of GABA(A)Rs associated to lipid rafts that is non-synaptic and that has a subunit composition different from that of the synaptic GABA(A)Rs. Some of the lipid raft-associated GABA(A)Rs might be involved in tonic inhibition.

High dietary cholesterol facilitates classical conditioning of the rabbit's nictitating membrane response

Studies have shown that modifying dietary cholesterol may improve learning and that serum cholesterol levels can be positively correlated with cognitive performance. Rabbits fed a 0, 0.5, 1 or 2% cholesterol diet for eight weeks and 0.12 ppm copper added to their drinking water received trace and then delay classical conditioning pairing tone with corneal air puff during which movement of the nictitating membrane (NM) across the eye was monitored. We found that the level of classical conditioning and conditioning-specific reflex modification (CRM) as well as the number of beta amyloid-labeled neurons in the cortex and hippocampus were a function of the concentration of cholesterol in the diet. The data provide support for the idea that dietary cholesterol may facilitate learning and memory.

Protein stability and the evolution of the cell membrane

Cholesterol has been shown to regulate the activity of several membrane proteins. Although this phenomenon represents an important factor in the regulation of ion homeostasis, insights are needed to fully understand the role of this lipid in cell function in order to better comprehend the effect of bilayer components upon membrane function. Since evolution has shaped the composition of the membrane bilayer, it becomes of interest to study these changes in parallel with the many functions of membranes such as ion transport. The present study employing a plasma membrane preparation obtained from calf ventricular muscle demonstrates that cholesterol partially inhibits the Ca(2+),Mg(2+)-ATPase as the catalytic function of the calcium pump, when incubation reaction temperatures are below 42 degrees C. In contrast, when incubation reaction temperatures are above 42 degrees C, cholesterol apparently promotes enzyme stabilization reflected in higher activity. Although the activation energy values for the enzyme are almost the same at ranges between 15 and 40 degrees C, the use of elevated temperatures promote higher enzyme inactivation rates in control than in cholesterol enriched membranes. Cholesterol apparently is promoting stabilization that in turn protects the enzyme against thermal inactivation. This protective effect is reflected in a decrease of inactivation rate values and energy released during enzyme catalysis. The modification of many membrane properties throughout million of years made it possible for new evolutionary driving forces to show themselves as new characteristics in eukaryotes such as the one discussed in this study, dealing with the presence of cholesterol in the cell membrane directly associated to the promotion of protein thermostability.

Mechanisms of general anesthesia

PURPOSE OF REVIEW

Anesthetics influence a wide variety of transmitter- and voltage-gated ion channels in the mammalian central nervous system. At the molecular level, the gamma-aminobutyric acid (GABA) subtype A receptor has emerged as a primary therapeutic target. This review highlights recent advances in our understanding of how anesthetics modify GABA(A) receptor function.

RECENT FINDINGS

Anesthetics bind to discrete selective binding sites on GABA(A) receptors--a discovery that challenges lipid-based theories of anesthesia. Not all GABA(A) receptors are equally sensitive to anesthetics because positive allosteric modulation is critically dependent on receptor subunit composition. Moreover, GABA(A) receptors located in extrasynaptic regions of hippocampal neurons display a greater sensitivity to propofol and benzodiazepines than do receptors located in subsynaptic regions. Enhancement in GABAergic inhibition may not account for all of the behavioral end-points associated with the anesthetic state. In particular, the immobilizing properties of anesthetics may not be solely mediated by GABA(A) receptors. Finally, synthetic neurosteroids are being developed as improved general anesthetics.

SUMMARY

Detailed insights into anesthetic-GABA(A) receptor interactions have resulted in intense efforts to develop safer drugs that selectively target subtypes of GABA(A) receptors.

Modulating inhibitory ligand-gated ion channels

The glycine and gamma-aminobutyric acid receptors (GlyR and GABA(A)R, respectively) are the major inhibitory neurotransmitter-gated receptors in the central nervous system of animals. Given the important role of these receptors in neuronal inhibition, they are prime targets of many therapeutic agents and are the object of intense studies aimed at correlating their structure and function. In this review, the structure and dynamics of these and other homologous members of the nicotinicoid superfamily are described. The modulatory actions of the major biological macromolecules that bind and allosterically affect these receptors are also discussed.

Effect of thiopental sodium on N-methyl-D-aspartate-gated currents

PURPOSE

N-methyl-D-aspartate (NMDA) receptors in the prefrontal cortex (PFC) are closely related with the excitability of pyramidal neurons and PFC function. As the effect of thiopental sodium on the central nervous system may partly result from the inhibition of PFC NMDA receptors, we investigated the effect of thiopental sodium with different concentrations on NMDA-gated currents in acutely dissociated rat PFC pyramidal neurons. We sought to determine whether thiopental sodium inhibits NMDA receptor function.

METHODS

Three to four week old male Sprague-Dawley rats were sacrificed and the PFC was dissected. Pyramidal neurons from the PFC were prepared and standard whole-cell patch clamp recordings were performed. Escalating concentrations from 3-1000 microM NMDA were applied 100 microm from the pyramidal cells, and the concentration in the effect compartment related to 50% effect (EC50) of NMDA was determined for the ensuing experiments. One hundred microM NMDA alone (control) or NMDA with different concentrations (10-1000 microM) of thiopental sodium were applied. After the inhibitory concentration, in 50% of NMDA effect (IC50) of thiopental sodium was established this IC50 and NMDA 3-1000 microM were applied 100 microm from the pyramidal cells. The EC50 value of NMDA under the effect of IC50 thiopental sodium was determined.

RESULTS

N-methyl-D-aspartate induced inward currents in a concentration-dependent manner, which were completely antagonized by 50 microM AP5. The maximal amplitude of NMDA-induced current was 1.15 +/- 0.27 nA. The EC50 of NMDA was 53.6 +/- 12.4 microM. The NMDA (100 microM)-gated current was inhibited by thiopental sodium in a concentration-dependent manner, and the IC50 of thiopental sodium was 33.6 +/- 6.1 microM. Under the effect of 33.6 microM thiopental sodium, the maximal amplitude of NMDA-induced current was 0.87 +/- 0.17 nA. The concentration-response curve of NMDA was shifted rightwards. The EC50 of NMDA was 128 +/- 15 microM, which was greater than that of NMDA without thiopental sodium (P < 0.01).

CONCLUSIONS

Thiopental sodium decreases NMDA-gated currents in acutely dissociated rat prefrontal cortical pyramidal neurons in a concentration-dependent manner.

Role of cholesterol in the function and organization of G-protein coupled receptors

Cholesterol is an essential component of eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The modulatory role of cholesterol in the function of a number of membrane proteins is well established. This effect has been proposed to occur either due to a specific molecular interaction between cholesterol and membrane proteins or due to alterations in the membrane physical properties induced by the presence of cholesterol. The contemporary view regarding heterogeneity in cholesterol distribution in membrane domains that sequester certain types of membrane proteins while excluding others has further contributed to its significance in membrane protein function. The seven transmembrane domain G-protein coupled receptors (GPCRs) are among the largest protein families in mammals and represent approximately 2% of the total proteins coded by the human genome. Signal transduction events mediated by this class of proteins are the primary means by which cells communicate with and respond to their external environment. GPCRs therefore represent major targets for the development of novel drug candidates in all clinical areas. In view of their importance in cellular signaling, the interaction of cholesterol with such receptors represents an important determinant in functional studies of such receptors. This review focuses on the effect of cholesterol on the membrane organization and function of GPCRs from a variety of sources, with an emphasis on the more contemporary role of cholesterol in maintaining a domain-like organization of such receptors on the cell surface. Importantly, the recently reported role of cholesterol in the function and organization of the neuronal serotonin(1A) receptor, a representative of the GPCR family which is present endogenously in the hippocampal region of the brain, will be highlighted.

Allopregnanolone treatment, both as a single injection or repetitively, delays demyelination and enhances survival of Niemann-Pick C mice

Niemann-Pick C disease (NPC) is an irreversible neurodegenerative disorder without current treatment. It is thought to result from deficient intracellular cholesterol and/or ganglioside trafficking. We have investigated the effects of allopregnanolone treatments on survival, weight loss, motor function, magnetic resonance imaging (MRI), and neuropathology in the mouse model of NPC (Npc1(-/-) mice). We confirmed previous results showing that a single injection of 250 microg of allopregnanolone on postnatal day 7 significantly extended the life span of Npc1(-/-) mice. This caused a marked difference in the weight curves of the treated mice but no statistical difference in the Rota-Rod performance. T2-weighted MRI and diffusion tensor imaging (DTI) of treated mice showed values of signal intensity and fractional anisotropy closer to those of wild-type mice than those of untreated Npc1(-/-) mice. Neuropathology showed that day-7 treatment markedly suppressed astrocyte reaction and significantly reduced microglial activation. Furthermore, the steroid treatment also increased myelination in brains of Npc1(-/-) mice. Similar effects of allopregnanolone treatment were observed in Npc1(-/-), mdr1a(-/-) double-mutant mice, which have a deficient blood-brain barrier, resulting in increased steroid uptake. The effects on survival and weight loss of a single injection on day 7 followed by injections every 2 weeks were also evaluated in Npc1(-/-) mice, and the beneficial effects were found to be greater than with the single injection at day 7. We conclude that allopregnanolone treatment significantly ameliorates several symptoms of NPC in Npc1(-/-) mice, presumably by effects on myelination or neuronal connectivity.

Cholesterol and synaptic transmitter release at crayfish neuromuscular junctions

During exocytosis of synaptic transmitters, the fusion of highly curved synaptic vesicle membranes with the relatively planar cell membrane requires the coordinated action of several proteins. The role of membrane lipids in the regulation of transmitter release is less well understood. Since it helps to control membrane fluidity, alteration of cholesterol content may alter the fusibility of membranes as well as the function of membrane proteins. We assayed the importance of cholesterol in transmitter release at crayfish neuromuscular junctions where action potentials can be measured in the preterminal axon. Methyl-beta-cyclodextrin (MbetaCD) depleted axons of cholesterol, as shown by reduced filipin labelling, and cholesterol was replenished by cholesterol-MbetaCD complex (Ch-MbetaCD). MbetaCD blocked evoked synaptic transmission. The lack of postsynaptic effects of MbetaCD on the time course and amplitude of spontaneous postsynaptic potentials or on muscle resting potential allowed us to focus on presynaptic mechanisms. Intracellular presynaptic axon recordings and focal extracellular recordings at individual boutons showed that failure of transmitter release was correlated with presynaptic hyperpolarization and failure of action potential propagation. All of these effects were reversed when cholesterol was replenished with Ch-MbetaCD. However, focal depolarization of presynaptic boutons and administration of a Ca2+ ionophore both triggered transmitter release after cholesterol depletion. Therefore, both presynaptic Ca2+ channels and Ca2+-dependent exocytosis functioned after cholesterol depletion. The frequency of spontaneous quantal transmitter release was increased by MbetaCD but recovered when cholesterol was reintroduced. The increase in spontaneous release was not through a calcium-dependent mechanism because it persisted with intense intracellular calcium chelation. In conclusion, cholesterol levels in the presynaptic membrane modulate several key properties of synaptic transmitter release.

Cholesterol homeostasis failure as a unifying cause of synaptic degeneration

We previously showed that fine tuning of neural cholesterol dynamics is essential for basic synapse function, plasticity and behavior. Significant experimental evidence indicates that cholinergic function, ionotropic and metabotropic receptor machinery, excessive tau phosphorylation, the change of amyloid beta (Abeta or Abeta) biochemistry, neural oxidative stress reactions, and other features of neurodegeneration also depend on fine tuning of brain cholesterol homeostasis. This evidence suggest that (i) cholesterol homeostasis break is the unifying primary cause of sporadic and familial Alzheimer's disease (AD), neuromuscular diseases (particularly inclusion-body myositis), Niemann-Pick's type C disease and Down syndrome, and (ii) explains the overlap of neurodegenerative hallmarks across the spectrum of neurodegenerative diseases. Provided is evidence-based explanation of why extremely rare (but scientifically popular) cases of AD associated with mutations in amyloid beta protein precursor (APP) and presenilin (PS) genes, are translated into the disorder via membrane cholesterol sensitivity of APP processing by secretases and Abeta generation. The reciprocal effect of Abeta on cholesterol synthesis, cellular uptake, efflux and esterification is summarized, as well as the potential implication of such biological function for the compensatory Abeta-assisted restoration of the synaptic long-term potentiation (LTP) and resulting inability of tackling amyloid to cure AD.

Hippocampal and striated skeletal muscle changes in fatty acid composition induced by ethanol in alcohol-preferring rats

Chronic ethanol intake affects various organ systems of the body. The present study evaluated modifications of fatty acid concentrations both in brain and striated skeletal muscles of rats genetically selected for voluntary high ethanol intake. Three groups of rats were tracked for 10 weeks of access to ethanol only as fluid (group 1) to free choice of ethanol and water (group 2) or to water only (group 3). At the end of the period, the animals were sacrificed and their brain hippocampus and striated skeletal muscles were removed and fatty acid content of these tissues was determined. Long-chain fatty acid content increased in the hippocampus while it decreased in the striated skeletal muscles. Short chain fatty acid content decreased in the hippocampus while short chain fatty acid content increased in the striated skeletal muscles. The data show that brain and striated skeletal muscles differently modulate fatty acid content perhaps because these areas utilize different cell membrane functionality regulation systems.